The present invention relates to a variable impedance network. More particularly, the invention relates to such a variable impedance network with coarse and fine controls.
Variable impedance networks are usually manually adjusted to provide a selected impedance so as to affect some aspect of the circuit in which the networks are located. These variable impedance networks are usually in the form of variable resistors, also called potentiometers. However, circuits using variable inductors or capacitors may also be formed.
Manual adjustment of potentiometers is usually undesirable in circuits under the control of data processing systems or other external electric circuits where ongoing adjustment of the potentiometer is necessary for circuit operation. The data processing system often must change the value of the variable impedance network in a time that is short relative to the time required to complete a manual adjustment of the variable impedance element. Therefore, special purpose integrated circuit variable impedance networks have been employed in the prior art. These networks allow the level of attenuation to be adjusted under the digital control of an external data processing system.
For example, Tanaka, et al., U.S. Pat. No. 4,468,607, teaches a ladder attenuator which is controlled by a binary number by means of a switch circuit. Depending on the stage of the switches in this switch circuit, one or more stages of attenuation are introduced into the signal path. However, teachings of Tanaka may require a large number of fixed impedance elements and switches for a large range of impedances. Accordingly, Drori, et al., U.S. Pat. No. 5,084,667, suggests a number of embodiments of variable impedance elements which minimizes the number of separate resistors required to achieve the equivalent resolution achievable using a series arrangement of resistors.